Method for sealing a ceramic molding

ABSTRACT

The present invention provides a method for encapsulating a ceramic moulding, preferably a silicon ceramic moulding such that the encapsulated mouldings can be subjected to hot isostatic pressing without causing the pressure-transfer medium to penetrate into the pores of the moulding. Encapsulation is achieved by first evacuating the moulding, then filling it with nitrogen gas and thereafter immersing it in a silicon (Si) melt. In a preferred aspect the moulding is filled with nitrogen gas under positive pressure. Once the silicon melt has been deposited, pressure can be allowed to act on it externally.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for sealing a ceramic molding,especially a silicon ceramic molding, for hot isostatic pressing. In thehot isostatic pressing (HIP) process, porous mouldings of a siliconceramic material are compressed under high pressure acting all aroundand under high temperature. Hot isostatic pressing reduces the porosityof the moulding and so improves its mechanical strength at elevatedtemperatures as well as its resistance to oxidation. The pressures usedin hot isostatic pressing run in the vicinity of 3000 bars, thetemperatures at about 1750° C. Use is normally made of a gas as a mediumto transfer the pressure. In order to keep this gas from penetratinginto the pores of the molding, the latter requires encapsulating.

2. Discussion of the Prior Art

Encapsulation is currently being practiced such that the ceramic moldingis enclosed in an evacuated glass can which turns viscous during hotisostatic pressing to hug the ceramic molding as a layer of glass.

With this known method for encapsulating the ceramic molding, the glasslayer threatens to penetrate into the pores of the molding. Should themelting glass exhibit too low viscosity during hot isostatic pressing,the strength of the molding may be compromised at its surface zone.Another risk is that the glass, if of low viscosity, may penetrate intothe pores of the molding to an extent where the glass envelope willintimately unite with the molding such that when the glass envelope isbeing removed after hot isostatic pressing, damage to the ceramicmolding cannot be avoided especially if it is a complex, thin-walledshape. On the other hand there is the risk that the glass can, whenmelting during hot isostatic pressing, and the resulting glass layerhugging the ceramic moulding have excessive viscosity and deform themoulding, which should definitely be avoided.

U.S. Pat. No. 4,230,745 discloses a method of encapsulating a moldedceramic member wherein the molded member is first filled with N₂, andthen coated with a dense surface coating of Si melt and thereafterexposed to an N₂ atmosphere until the Si coating is converted into a Si₃N₄ coating. In contrast, the present invention provides a method ofsealing a ceramic molding without forming a surface layer of glass toencapsulate the molding. The pores are sealed with plugs of Si₃ N₄. Thisimprovement retains the original geometric shape, eliminates thepossibility of a mistake in forming an external layer, and thepossibility of trapping the metallic silicon residue between the coatingand the molding.

SUMMARY OF THE INVENTION

In a broad aspect the present invention provides a method for sealing aceramic molding to seal out the pressurized gas while maximallyretaining the original geometric shape of the ceramic molding.

It is a particular object of the present invention to provide a methodin which the molding is first evacuated and then filled with nitrogengas before it is immersed in a silicon melt.

DISCUSSION OF THE PREFERRED EMBODIMENTS

Immersion of the molding in the silicon melt causes liquid silicon topenetrate into the pores of the molding and to react with the nitrogengas in the molding to form Si₃ N₄. The resulting Si₃ N₄ plugs seal thepores and give the molding a gas-tight surface permitting compression byhot isostatic pressing.

In a preferred aspect of the present invention the molding and the meltare heated to a temperature of 1400° C. to 1800° C. before immersion toensure adequate wetting of the moulding with the liquid silicon.

In a further aspect of the present invention the molding is filled withpressurized nitrogen, which measure serves to inject a maximum amount ofnitrogen gas into the molding, permitting maximum growth of the Si₃ N₄plugs after the silicon melt has penetrated.

In a preferred aspect of the present invention the pressure on thesilicon melt is raised steadily after immersion of the molding, causingthe developing Si₃ N₄ skin within the pores to be ruptured continuallyand to be formed anew farther into the pores.

In a further aspect of the present invention the pressure on the siliconmelt is raised and lowered periodically upon immersion of the molding.This causes the Si₃ N₄ skin within the pores to be ruptured continuallyuntil the fragments grow together to form a solid plug. This lattermethod enables thicker Si₃ N₄ plugs to be produced than with the methodof the former aspect. Another parameter immediately governing thethickness of the Si₃ N₄ plugs is the pressure at which the nitrogen isforced into the still porous molding. The higher the pressure selected,the more plentiful the supply of nitrogen in the molding and the thickerthe Si₃ N₄ plugs.

What is claimed is:
 1. Method for sealing a ceramic molding withoutencapsulating or forming a surface layer, especially a silicon ceramicmolding for hot isostatic pressing, characterized in that the molding isfirst evacuated and then filled with nitrogen gas (N₂) before it isimmersed in a silicon melt, said immersion thereby forming Si₃ N₄ plugswithin the pores to seal the pores and thereby obviate the need for asurface layer coating before said isostatic pressing.
 2. Method of claim1, characterized in that the molding and the melt are heated to atemperature of 1400° C. to 1800° C. before immersion.
 3. Method of claim1 or 2, characterized in that the filling of the molding is achievedusing pressurized nitrogen gas (N₂).
 4. Method of claim 1 or 2,characterized in that the pressure on the Si melt is steadily raisedupon immersion of the molding.
 5. Method of claim 1 or 2, in which themoulding is filled with pressurized nitrogen gas (N₂), characterized inthat the pressure acting on the silicon melt is raised steadily uponimmersion of the molding.
 6. Method of claim 1 or 2, characterized inthat the pressure acting on the silicon melt is raised and loweredperiodically upon immersion of the molding.
 7. Method of claim 1 or 2,in which the molding is filled with N₂ under positive pressure,characterized in that the pressure acting on the Si melt is raised andlowered periodically upon immersion of the molding.
 8. A method ofsealing a silicon ceramic molding without encapsulation to protect itduring hot isostatic pressing, said method comprising:(a) evacuating themolding, (b) filling the molding with nitrogen gas (N₂), and then (c)immersing the nitrogen filled molding in a silicon melt, whereby thesilicon melt reacts with nitrogen gas within the pores of the molding toform Si₃ N₄ plugs that close and seal the pores of the molding duringsubsequent hot isostatic pressing, without forming a surface layer ofSi₃ N₄ around the molding.
 9. A method according to claim 1 furthercharacterized by the step of maintaining the surface of the moldingsubstantially free of Si₃ N₄ as the Si₃ N₄ plugs are formed in the poresof the molding.
 10. A method according to claim 8 further comprising thestep of maintaining the outside surface of the molding substantiallyfree of Si₃ N₄ during the immersing step.